1. Field of the Invention
This invention relates generally to wireless communication systems and more particularly to supporting multiple wireless communication protocols within a wireless local area network using a long training sequence.
2. Description of Related Art
Wireless and wire lined communications between wireless devices and components may use networks or systems to exchange data or information. Communication systems may include national or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Communication systems and networks may operate in accordance with one or more communication protocol standards. For example, wireless communication systems may operate in accordance with one or more standards including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and the like.
A wireless communication device may comply with a protocol or standard for a wireless communication system. The appropriate standard for wireless communications may vary. For instance, as the IEEE 802.11 specification has evolved from IEEE 802.11 to IEEE 802.11b (standard 11b) to IEEE 802.11a (standard 11a) and to IEEE 802.11g (standard 11g), wireless communication devices that are compliant with standard 11b may exist in the same wireless local area network (WLAN) as standard 11g compliant wireless communication devices. As another example, standard 11a compliant wireless communication devices may reside in the same WLAN as standard 11g compliant wireless communication devices.
The different standards may operate within different frequency ranges, such as 5 to 6 gigahertz (GHz) or 2.4 GHz. For example, standard 11a may operate within the higher frequency range. One aspect of standard 11a is that portions of the spectrum between 5 to 6 GHz are allocated to a channel. The channel may be 20 megahertz (MHz) wide within the frequency band. Standard 11a also may use orthogonal frequency division multiplexing (OFDM). OFDM may be implemented over subcarriers that represent lines, or values, within the frequency domain of the 20 MHz channels. A signal may be transmitted over many different subcarriers within the channel. The subcarriers are orthogonal to each other so that information may be extracted off each subcarrier about the signal.
When legacy devices reside in the same WLAN as devices compliant with later versions of the standard, a mechanism may be employed to insure that legacy devices know when the newer version devices are utilizing the wireless channel to avoid a collision. For example, backward compatibility with legacy devices may be enabled at the physical (PHY) layer, as in the case of standard 11b, or the Media-Specific Access Control (MAC) layer, as in the case of standard 11g. At the PHY layer, backward compatibility may be achieved by re-using the PHY preamble from a previous standard. In this instance, legacy devices may decode the preamble portion of all signals to provide sufficient information for determining that the wireless channel is in use for a specific period of time, thereby avoiding collisions even though the legacy devices cannot fully demodulate or decode the transmitted frame(s).
Backward compatibility with legacy devices also may be enabled by forcing devices that are compliant with a newer version of the standard to transmit special frames using modes or data rates that are employed by legacy devices. For example, the newer devices may transmit Clear to Send/Ready to Send exchange frames or Clear to Send to self frames as may be employed in standard 11g. These special frames contain information that sets the network allocation vector of legacy devices such that these devices know when the wireless channel is in use by newer stations.
These mechanisms for backward compatibility may suffer from a performance loss relative to that which can be achieved without backward compatibility and are used independently of each other. Further, in standard 11a and 11g transmitters, only 52 subcarriers (−26 . . . −1 and +1 . . . +26) may be filled with non-zero values even though an inverse fast Fourier transform (IFFT) of length 64 is used. As such, sharp frequency-domain transitions occur between zero subcarriers and non-zero subcarriers, which results in a time-domain ringing. Time-domain ringing may adversely affect a receiver's ability to detect a valid preamble transmission and require the receiver to perform a channel estimate using the full fast Fourier transform (FFT) size. Further, the estimation of the channels at the receiver may be compromised.
Long training may be performed for channel sounding and estimation in legacy and current systems. One action to estimate a channel may be to stimulate all the subspaces of the channel, or close to all, such as 63 out of 64 subcarriers. In a long training sequence, each of the subchannels may be stimulated with a signal to obtain a simplified least squares estimate on the receiver side. A drawback to this action may be that if you stimulate all the subcarriers, except the zero (0) subcarrier, the spectral mask requirements may not be met. If the spectral mask requirements are not met, excessive channel interference may be generated.